How can I easily customize my email subject lines using the Mail Merge function?

Map the Email Subject to a header column in your spreadsheet (1).

Add the common data in the cell below the header column and use a formula (2) to pull in custom data from another cell in your spreadsheet. Copy and paste your formula to cells on rows where you wish generate a Mail Merge. 

In the case below the formula we will use is

="Training program for "&E2

Only copy formulas to cells where you wish to generate a doc and/or mail merge as the add-on will run on every cell where data is detected. 

Or use a row filter on a criteria column that does not have a formula, like "date >  Is Not Empty".